Determination of Selenium in Environmental Samples Using Gas Chromatography with a Microwave Emission Spectrometric Detection System Yair Talmi Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
Anders W. Andren Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
The applicability of a gas-chromatograph with a microwave emission spectrometric detector (GC-MES) to the determination of trace amounts of selenium in environmenlallybased samples is described. The analysis is based on chelating Se'" with 5-nitro-0-phenylene diamine (PD) to form the thermally stable and volatile piaselenol complex. Thls is followed by its extraction into toluene, separation by the GC, and, finally, determination by the MES via monitoring the emlssion intensity at the 204-nm selenium line. Various parameters affecting the formation and extraction of the piaselenol complex such as time dependence, composition of digestion solution, selenium loss, and oxidation of the PD reagent are discussed along with some potential interfering parameters such as matrix composition, oxidation stale of the selenium, and column deterioration. The detection limit for selenium is 40 pg and the relative sensitivity is 0.1 pg/ liter for water samples and 15 ppb for solid samples. Samples analyzed included biological and plant tissues, coal, fly ash, and scrubber solutions. The relative error ranged from 0-17.5% and the relative standard deviation from 2.29.5%.
T h e applicability of gas chromatography to inorganic analysis has been discussed in numerous publications (16). Basically, the analytical procedures involve the reaction of trace elements with chelating agents to form thermally stable volatile derivatives which are amenable t o gas-chromatographic separation. Most studies in this area have involved the use of the electron capture detector, which is exceptionally sensitive to halogenated organic compounds. Yet, when applied t o inorganic analysis the detector suffers from the following shortcomings. T h e sensitivity and selectivity are strongly dependent upon the nature of the chelate rather than the trace element; arid the high response of the detector to organic compounds with highly polar functional groups limits the selection of solvent systems and, hence, the applicability of the technique. Flame spectrometric detectors, on the other hand, have demonstrated attractive selectivity features but their sensitivity is rather poor and their range of applicability is limited by their inability to provide high excitation energy. The microwave emission spectrometric detector (MES) (1) R. W. Moshier and R. E. Sievers, "Gas Chromatography of Metal Chelates," Pergamon Press, Oxford, 1965. (2) G. P. Morie and T. R. Sweet, Anal. Chem., 37, 1552 (1965). (3) R. E. Sievers, B. W. Ponder, M. L. Monis, and R. W. Moshier, lnorg. Chem., 2, 693 (1963). (4) R. W. Moshier and J. E. Schwarberg, Talanfa, 13, 445 (1966). (5) R. Belcher, W. I . Stephen, I. J. Thompson, and P. C. Uden, J. lnorg. Nucl. Chem., 33, 1851 (1971). (6) M. L. Taylor and E. L. Arnold, Anal. Chem., 43, 1328 (1971).
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NO. 14,
introduced by McCormack et al. ( 7 ) offers both excellent sensitivity and selectivity characteristics. T h e GC-MES system has been used by Bache and Lisk to analyze various pesticides by monitoring the characteristic atomic emission lines of their heteroatoms, i.e., C1, Br, I, S, and P (8, 9). T h e applicability of the system was recently extended to H, D, 0 (IO), organo-mercury compounds (11, I2), and metal chelates (13, 14). T h e present study describes the applicability of GC-MES t o the quantitative determination of submicrogram quantities of selenium in various environmental samples. The analytical procedure utilizes the formation of 5-nitro-benzoselenadiazol (piaselenol) ( 1 5 ) followed by its extraction into toluene and the injection of this layer into the gas chromatograph. T o prevent carbon deposits on the quartz capillary walls, the plasma is ignited only after most of the toluene is eluted from the column. T h e selenium is detected by monitoring the emission intensity of the 204-nm selenium line.
EXPERIMENTAL Apparatus. The GC-MES system is basically the same as that described by McCormack, Tong, and Cooke ( 7 ) ,although numerous instrumental modifications have been incorporated (16). The
gas chromatographic unit utilized in this study is a modified Tracor MT-220. A Pyrex capillary splitter directs the GC effluent to a flame ionization detector, which serves as a non-selective detector, and to the MES detector. The MES detector is comprised of a highly transparent quartz capillary (Thermal American Fused Quartz Inc.) which conducts the effluent gas from the GC oven into the center of a tapered microwave cavity (7097-1001G1 and 7097-500 G1 from Raytheon Corp.). An argon microwave plasma discharge is formed by a 100-watt microwave generator (Scintillonics Model HV15A) and is confined within the boundaries of the quartz capillary. All contact of the GC separated components with any metallic surface is eliminated to avoid undesirable chemical interactions. The GC-MES operating conditions for selenium determination are described in Table I. Reagents. Reagent grade chemicals were used throughout. Doubly distilled nitric acid was used for low selenium content samples. Dilute selenite standards were prepared from a 1000-ppm stock solution of SeOn. A IO-ppm 75Se(rV) including stable selenium carrier was also prepared for tracer studies. The 4-nitro-0-phenylenediamine(PD) was prepared by dissolving 1 gram in 1M HC1 t o form (7) A. J. McCormack, S. C. Tong, and W. D. Cooke, Anal. Chem., 37, 1470 (1965). (8) C. A. Bache and D. J. Lisk, Anal. Chem., 33, 1757 (1966). (9) C. A. Bache and D. J. Lisk, Anal. Chem., 39, 1786 (1967). (IO) W. R. McLean. D. L. Stanton, and G. E. Penketh, Analyst (London),98, 432 (1973). (11) C. A. BacheandD. J. Lisk, Anal. Chem., 43, 1950(1971). (12) Y. Talmi and R. Mesmer, submitted to Wafer Res., 1974. (13) H. Kawaguchi, T. Sakamoto, and A. Mizuike, Talanta, 20, 321 (1973). (14) R. M. Dagnall. T. S. West, and P. Whitehead, Analyst (London),98, 647 (1973). (15) M. Tanaka and T. Kawashima, Talanta, 12, 211 (1965). (16) Y. Talmi and R . Crosmun. "Ecology and Analysis of Trace Contaminants," Progress Report J u n e 1972-January 1973, ORNL-NSF-EATC-1, March 1973.
DECEMBER 1974
Table 11. T i m e Dependence of the Piaselenol
Table I. GC-MES Operating Conditions
Complex Formation
Parameter
GC column length, ft" GC column packing Quartz capillary, i. d., 0.d. mmb C a r r i e r gasC C a r r i e r gas flow rate, ml/min Column temperature, "C Injector temperature, "C Microwave generator output, watts Monochromator settingd Slit height, mm Slit width, p Wavelength, nm Photomultiplier tube Photomultiplier voltage, V Opticse Lens focal length, m m Lens diameter, m m
.
4
4% SE-30 on 30/60
Reaction time, min
5
mesh chromosorb GHP 0.5, 6.5
10 20 30 40 60
Argon 95 190 200 35
cpm/mP a q u e o u solution
250 255 2 50 245 255 260
cpmjmlb toluene extract
96 recovery 1st extractidn
8020 9450 10100 11100 11890 11870
64 76 81 89 95 95
QThevolume of the aqueous solution was 50 ml. The given cpm values refer to the selenium content of the aqueous phase before extraction. b The expected radioactivity was 12,500counts per minute (cpm)/ml toluene, assuming 100% recovery.
6 35 204 RCA 1P28 6 50 100 50
0 F'yrex column dimensions, i.d. 5 mm; o.d. 6.5 mm. * Quartz capillary length, 16 inches. The plasma was operated at atmospheric pressure. d Jarrell-Ash 0.5-m Ebert mounting scanning monochromator with 1180 grooves/mm. Grating blazed at 190 nm. Plasma image was focused on the entrance slit.
a 1% solution. The PI) solution was extracted twice with 20-ml portions of toluene to remove organic impurities. A fresh solution was prepared once each week. Stock bromine-bromide redox buffer solution was prepared by adding 2.5 grams of KBr to 50 ml of saturated bromine water and the volume diluted to 100 ml with distilled water. Before use, this solution was diluted 1:lOOO with distilled water. Analytical Procedure. Ayueous Samples. Samples 10-100 ml, were transferred into a 125-ml Erlenmeyer flask and 0.5 ml concentrated HCI and 3 ml of redox buffer were added for every 10 ml of sample. The solution was heated gently at 80 " C until it became colorless. After adding 2 ml of the PD reagent, the solution was mixed and let stand for 40 minutes. Extraction of the piaselenol complex with 1-2 ml of toluene was accomplished by shaking for five minutes. One extraction was adequate (Table 11). A 1- to 10-pl aliquot of the toluene layer, dried over anhydrous NaZS04 was injected into the GC by means of a 10-pl microsyringe. Solid Samples (Wet Ashing). Samples in the 0.1- to 1.0-gram range were accurately weighed into a 100-ml volumetric flask equipped with an air condenser. The air condensers were made from ST 14/35 ground glass joints with jackets of woven asbestos tubing held in place by Teflon sealing tape. The length of the condenser was approximately 15 cm. Concentrated HNO:j, 10-25 ml, was added to the'flask and heated slowly during the initial stages of the digestion and finally at a hot plate temperature of 250 " C (see Table 111, footnote c ) for approximately 20-25 minutes. Concentrated HC104,5-8 nil, was then added and the solution was vigorously heated at a hot plate temperature of 360 " C until the appearance of white fumes. Heating was then continued for another 5 minutes. Upon cooling, approximately 5 ml of distilled water and 1 ml of concentrated HC1 were added. The solution was then vigorously scrubbed by air using a Pasteur pipet for at least 5 minutes. After dilution to 100 ml, a 25-ml aliquot was transferred to a separatory funnel, and 1 ml of concentrated HCI was added. After the addition of the PD reagent, the procedure described for aqueous samples was followed. Inorganic samples such as fly ash and slag were leached with t5 ml of HC104 followed by the same procedure as described by wet ashing.
RESULTS A N D D I S C U S S I O N T h e determination of trace amounts of selenium(1V) via its piaselenol complex has previously been studied using such techniques as molecular absorption and fluorescence
(I 7-21 ) and gas chromatography-electron capture (22-24 1. T h e following is a discussion of the significant parameters affecting the GC-MES technique when applied t o this analysis. F o r m a t i o n and Extraction of the Piaselenol Complex. T i m e Dependence. T h e time required for the quantitative formation of the 4-nitro-o- phenylenediamine (PD) piaselenol complex was carefully studied at 24 f 2 "C by means of 75Se measurements. Aliquots, 200 ~ 1 of, the 75Se (as selenite) standard solution were added t o 25 ml of water. T h e solution was then acidified t o p H
